1. Field of the Invention
The present invention relates generally to diffraction gratings, and more specifically to the manufacture of multilayer diffraction gratings.
2. Discussion of the Related Art
Diffraction gratings are integral parts of chirped-pulse amplification (CPA) high peak power lasers. In this scheme, high peak power is achieved by creating a laser pulse which contains an energy density near the saturation fluence of the laser medium and a duration near the inverse of its gain narrowed bandwidth. Diffraction gratings are used to distribute the energy of the laser pulse over a much longer time period prior to amplification in order to prevent damage to the laser amplifier. Following amplification, the pulse is compressed to extremely high power density by passage through a pulse compressor based on diffraction gratings. Diffraction gratings are the limiting factor to the power achievable with these systems, since a final grating must withstand this high power density in order to reform the short pulse. Therefore, for future higher power and energy laser systems it is desirable to both increase the size (area) and damage threshold of the gratings. The metallic diffraction gratings that were used in the world's most powerful laser system, the petawatt, were required to be nearly one square meter in area in order to handle the laser power. Lasers with increasing power and energy levels are required for many important experiments in high energy density physics and inertial confinement fusion.
Prior art designs and fabrication methods for advanced gratings that use non-metallic materials have inherently higher laser damage threshold that prior metallic gratings. For example, in U.S. Pat. No. 4,915,463 (which is incorporated herein by reference), Barbee, Jr., a fabrication method for a grating of this type is described where the grating pattern is etched into a bulk substrate, prior to the coating of a reflecting dielectric multilayer on the surface. This grating is impractical for the CPA application since the high efficiency and large chromatic bandwidth required for the CPA application means that the multilayer would have to contain many layers (>20) to have high reflectivity. This large number of layers will act to planarize the patterned surface, destroying the grating structure. In U.S. Pat. No. 5,119,231 (which is incorporated herein by reference), Nelson et al. describe an etching step, with a planarization layer of different refractive index before applying a multilayer coating to the surface. This solution would also not be applicable due to many reasons. The planarization layer will cause additional evanescent losses for the diffraction grating limiting the efficiency. Also, the incident beam would have to travel through the substrate, which is not practical for short, high peak power pulses due to the potential for damage to the substrate resulting from self-focusing.
In U.S. Pat. No. 5,907,436 (which is incorporated herein by reference), Perry et al. provide a multilayer grating that is designed specifically for the pulse-compression grating application. A grating pattern is formed in the top layer of a multilayer dielectric coating that has high reflectivity. The multilayer dielectric coating is formed on an insulating substrate. Perry et al. suggest a large number of fabrication options, including dry etching, wet etching and lift-off processing. Furthermore, U.S. Pat. No. 4,313,648 (Yano et al.) and U.S. Pat. No. 5,510,215 (Prince et al.) (both of which are incorporated herein by reference) describe the patterning of multilayer dielectric stripe filters including fabrication methods for producing patterned dielectric multilayer stacks using dry etching.
According to known fabrication techniques for multilayer diffraction gratings, insulating materials, such as fused silica (glass), are predominantly used as the substrate material upon which the multilayer dielectric structure is attached. Such materials provide good insulating properties and good adherence to the oxide dielectric layers formed thereon.
The primary fabrication choice for the patterning of multilayer dielectrics, such as described in the Perry, Yano and Prince patents above, is dry etching, notably reactive ion etching (RIE). Reactive-ion etching is favored over many different etch methods due to its high selectivity and anisotropic characteristics. As illustrated in FIG. 1, in reactive ion etching, a plasma excites gases that chemically attack the target material. Ions within the plasma are accelerated towards the substrate by a DC bias voltage applied at the bottom of the substrate. These ions can excite or ionize other species which then chemically react with the substrate or, they can attack the substrate directly. It is the combination of chemical and physical processes that gives RIE high anisotropy and selectivity.
However, large gratings (to satisfy the requirements of high peak power laser systems) require that the substrate is thick due to the desire to maintain wavefront quality of the gratings. This is due to stresses in the coating or even the weight of the substrate itself being able to deform the substrate to an extent that it is unusable unless the thickness of the substrate is sufficient to prevent this distortion. A typical aspect ratio that is used for the maximum width of the substrate compared to the thickness is 6:1. Hence, gratings of sizes greater than 40 cm in width, would be required to be greater than 6.5 cm thick to satisfy this criterion.
Descriptions of diffraction gratings and methods to fabricate them, such as described in the Perry, Yano and Prince patents, do not consider the thickness of the substrate material. For small-size gratings in which the thickness of the substrate is inherently thin (less than 0.5 cm) and for gratings fabricated with ion-beam etching processes, the substrate thickness is not important, However, reactive ion etching has proven challenging in the fabrication of large size multilayer diffraction gratings in which the substrate is thick, e.g., at least 2.0 cm. Accordingly, instead of using reactive ion etching, such diffraction gratings are typically fabricated using ion beam etching, which uses a focused ion beam to directly ablate the target surface.
In one approach, such as described in U.S. Pat. No. 5,296,091 to Bartha et al. (which is incorporated herein by reference), the etching of substrates having a low thermal conductivity is improved by providing a cooling apparatus to remove the excess heat generated at the substrate in order to minimize non-uniform heating effects.